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What kind of airspeed does a (3D) GPS measure?
Assuming a GPS receiver is advanced enough to compute 3D position/velocity with reasonable accuracy, it can indicate not only the aircraft's ground speed (which ignores altitude changes), but something akin to airspeed as well. If there was no wind, this would be equivalent to true airspeed. In reality there would be wind so it is "TAS minus wind". Since this completely ignores the actual air (the value would have been the same if there were no atmosphere), however, calling it anything related to "airspeed" seems like a misnomer to me.
Is there a standard terminology for this kind of velocity measurement?
EDIT:
It seems my question was quite vague and providing more detail about what I'm trying to do would help. I'm writing the documentation for a GPS-based tracking device, which would be used in fixed-wing aircraft. The GPS is exceptionally good, with very high precision and quick sampling rate. It even provides very accurate speed, which is Doppler-shift based AFAIK. This data will be sent through telemetry. I'm trying to label the data channels in a way that would be useful for aviators down on the surface, so I'm trying to familiarize myself with the proper terminology. Therefore if I label this ("3D") velocity as "airspeed" (indicated/true airspeed), it will be misleading. If I label it "ground speed" (which is 2D), it will be misleading as well. My question is only whether a standardized name for this type of velocity exists; if there's nothing widely used, I'll just coin a name of my own (e.g. "3D velocity") and explain it well enough in a tooltip.

You have confused terms. True Airspeed is not dependent on wind. TAS is airspeed adjusted for air density. It is adjusted from Indicated Airspeed factoring in altitude and temperature. Airspeed in general is based on the aircraft’s movement through an airmass regardless of wind. GPS calculates velocity based on the difference in position over time relative to the GPS transmitters. It does not factor in movement through an airmass nor air movement at all. Calling it’s readings with no data from air movement would technically not be any type of airspeed.

@DeanF., yeah, exactly, as I said it's tempting to call it airspeed (because it won't show as 0 if the airplane was falling straight down), however there's no air involved. It is velocity relative to the Earth's surface.

If you understand that in zero wind TAS = GS, then what are you actually asking?!

Do you really mean to ask, "What kind of velocity does GPS measure?"

@RalphJ, yes, and more exactly "how to you typically call this kind of velocity in aviation context"

Regarding your edit:  Who is your customer, and what is the purpose of this device?  Existing aircraft GPS can store and transmit track data.

I can't disclose much details about the project I'm working on, due to confidentiality agreements. But yes, we compared it to an existing Garmin GPS on the plane, which was mediocre in comparison. Also the device records much more than just GPS, but again, it's confidential.

What we have is the 3D spd (velocity) resolved into E/W and N/S (= Track and Groundspeed,  and UP/DN (= Vertical. Spd, normally used to validate baro VS). This supports then traditional modes of flying instrumentation. Pls see my detailed answer. esp. the final para which addresses your need.

Can we take it that the data you're talking about is from fixed wing aircraft and only to be used by a ground based crew? Is it to do with the recent mandate for aircraft tracking at 15min intervals ( later to be minute by minute)? Is there a need for the end users to know these details? Is it going to be used for realtime terrain avoidance? With a 3D map (atlas) database, a suitable moving 3D map with airplane path - desired and actual - can easily be rendered. Don't know if then 3D velocity vector even needs a name apart from it's existing data tag.

@skipper44, sorry, I cannot disclose any of that. But as we already agreed with Michael Hall, it would be most useful to split the velocity in horizontal and vertical components. I understand 3D velocity is not very useful as it is.

I decided to just go ahead and ask the question I had in mind, rather than trying to re-write yours. Will probably lead to discussion about whether or not they are duplicates...

Q: "What kind of airspeed does a GPS measure?"
A: "None whatsoever."

The GPS does not sense or compute any type of airspeed, (True, indicated, calibrated, etc.) it only knows the airplane's velocity relative to the earth's surface.  (Actually, relative to geosynchronous satellites...) GPS units currently in use can do this in 3 dimensions.
There is both a horizontal and a vertical component to movement in 3D space.  For general navigation purposes the pilot is only concerned with the horizontal, i.e. ground speed.  (How fast am I going, and when will I get there?)
On some systems you can display vertical velocity from the GPS, but it is instantaneous and very jumpy, so it isn’t particularly helpful.  The pressure driven VSI gauge is much more useful.
While it would be possible for a 3D system to compute velocity along a diagonal line between vertical and horizontal, this information is of no use to the pilot.  The vertical component comes into play mainly in the approach phase, to compute and display glide slope information during RNAV approaches.
In the approach phase the GPS system uses a blend of horizontal and vertical navigation information to allow us to fly the “hypotenuse”, but we don’t think of it in terms of a velocity vector, much less an “airspeed”.   It is more an imaginary line in space we are trying to track along in 3D with the system providing cues to help us know where we are relative to the published course and glideslope.
ADDENDUM PER QUESTION EDIT:
Wind will have a far greater effect on this velocity vector than the minor climb and descent angles encountered during "normal" (non-aerobatic) flight.  Without knowing your customer or exact purpose of the device you reference, I can say confidently that no aviation person would care about earth referenced velocity other than vectors in the horizontal or vertical.  If the diagonal, (not airspeed) velocity was desired for something like a mishap investigation, it could easily be calculated from others.  If you need to assign a name I would consider "Velocity", "Magnitude", "Earth-speed", or a proprietary defined acronym like "VVGS" (Vertical Velocity plus GroundSpeed).  Just avoid "air" unless that's what you actually mean...

Isn't ground speed 2D? I.e. if the aircraft is in 90° dive, what would it indicate?

Ground speed would be zero.  Vertical velocity, however, would be pegged!  But thanks, that clarifies what you are asking about and I will edit my answer...

OK, so it seems a good call to request the speed indication to be split into ground speed (2D) and rate of climb, right? (I agree you can compute the 3D velocity easily from those)

Again, without knowing the customer or purpose I would say yes.

"Just avoid "air" unless that's what you actually mean" -- well put.

Typically it is the Inertial based systems that give us this kind of data. A GPS with a suitable computing module could provide the same thing.
The closest thing we have is called the FPV, Flight Path Vector or the 'Bird'.
In the cockpit, generally, the FPV provides the pilot only a part of the information, and displays it in a particular manner, vertical path and Lateral path being separately shown.
The actual 3D speed component is not one of them, and neither would the 3D Speed be of direct value in flying, or navigating the airplane.
Typically it gives us:
(i)the instantaneous Track and Vertical angle of the actual flightpath with respect to the earth, ie the ground,
(ii)also often, it gives use a projection of where the airplane will be in 10, 20, and 30 seconds based on the current instantaneous flightpath.
(iii) resolving the East/West and North/South vectors gives us the instantaneous Track and Groundspeed.
(iiii) resolving the Vertical component of the FPV gives us the Vertical Speed, though it is normally only used to validate the Air Data based Barometric VertSpd, VS.
None of these systems actually measure any Airspeed. They measure GS, Ground Speed, though strictly speaking, it could not be called an act of measurement, but something arrived at by using position data, ie Lat/Long at suitable intervals and computing the difference to give an accurate estimation of Ground Track and GS (=TAS in still air).
The plethora of acronyms related to speed, direction, and vertical disposition were necessitated due to way we physically measure, then compute, then display them and our basic flying skills have developed around these - IAS, CAS, EAS, TAS, GS, ALT, FL, TRK CRS, HDG(M), HDG(T), ATT. There's no point introducing new measures such as 3D speed, though I'm guessing your intent maybe to have "one speed to rule them all" along with a suitable depiction of airplane and flightpath in a 3D EVS (Enhanced Vision System).

Thank you for the detailed answer. It seems this GPS-derived velocity serves no use to a pilot, so an instrument showing it has no space in a cockpit. My question is not really about airplane instrumentation though, it's mostly about naming and terminology. My question didn't state that clearly enough. Can you see the edit I made to it?

Re your edit: it's not so much as "one speed to rule them all", rather than "the only speed we can get". The device has no access to the pitot tubes.

For completeness, since it doesn’t appear to be spelled out anywhere else, the speed indication from a GPS is typically the speed in the horizontal plane, vertical speed isn’t taken into consideration.

@Frog A few decades ago the vertical speed displayed in the Airliner cockpit was modified to include  Inertial inputs and the nomenclature was changed to IVSI (Instantaneous Vertical Speed). The "Instantaneous" part has been dropped now, as it has become the 'new normal'. Pure baro VSI's suffered delays in displaying the correct VS. Whether such input corrections are specifically taken from GPS systems is not known to me, but the systems have the capability of providing it to us. My current airplane display can be switched to GPS altitude as an alternate when Air Data is unreliable.

"Pure baro VSI's suffered delays in displaying the correct VS."-- actually, traditional VSI's are intentionally designed to average the vertical speed over a substantial number of seconds.  Sailplane-style "variometers" provide a much quicker response even when driven purely via barometric means, with no inertial inputs.  Of course the latest and greatest variometers incorporate accelerometers as well, to provide near-instantaneous response.

To clarify: I’m referring to the NMEA feed from a GPS and specifically the RMC sentence.  I don’t dispute that certain GPS-equipped instruments may indicate speed in  3 dimensions.  GPS does not resolve altitude well and so it’s no surprise to hear that other methods such as inertial are used to determine vertical velocity.


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Is there any theoretical problem powering the fan with an electric motor
A question that I have been wondering about lately is whether it would be possible to power the fan in a modern hi-bypass turbofan with an electric motor and achieve performance comparable to the kerosene-fueled engine. Or is the 10% of air that flows through the combustion chamber crucial for the operation of the system?
(I wish to ignore all things that make electric aircraft unviable, like power-to-weight ratio of engines and battery capacity)
Edit: I have realized that the "electric" part of the question is really not the core of the question. What I am really interested in is whether it would be feasible to power a plane using only the fan and not the exhaust jet with a performance similar to a current jet (which would make the engine similar to a turboprop with many blades and a cover around it)

What would power the electric motor?  If you wish to ignore the weight of batteries you would need an engine driven generator... do you think this would be somehow more efficient than driving the fan directly?

I don't understand how you can "ignore all things that make electric aircraft unviable, like power-to-weight ratio of engines and battery capacity" ... if the plan won't fly because of the weight of the engines to provide the power, or because the number of batteries carried would cause the weight to be too much, then it's not a plane, it's a brick that can't take off.

Why do you think that "the 10% of air that flows through the combustion chamber" could/would not still flow through?

@erikkallen maybe a better question (or a follow up for this one) would be which thrust apparatus would be most suitable for electric airplanes: ducted fan, propeller, unducted fan etc. Electric motors have some characteristics that differ from turbines and piston engines, and taking these characteristics into account might tip the scales to a surprising direction. The application is of course one variable: what kind of missions would the plane need to fly etc.

See my comment on the latest answer with the embedded video.  It is not clear at all what you are actuall asking, and your edit only makes it worse.  Voting to close until you can clarify what you really mean.

This is irrelevant question if you ignore the batteries. Using batteries on a large installation would be - ah - ignorant. Wait for fusion to become viable. Avalanchee (https://avalanchefusion.com/) works on a 100kw reactor that sits on a normal desk - here comes the power.

Taking into account your wish to ignore all things that make electric aircraft unviable, like power-to-weight ratio of engines and battery capacity, the answer is yes.
You would, however, need to compensate the missing jet engine thrust by increasing the fan thrust.
Otherwise the answer is no.

Should someone feel irritated by this answer, you're wellcome! Throwing reality out the window does not spawn very good deliberations

Nobody should be irritated by truth.  Sometimes these random and theoretical musings just need a blunt debunking.

Any irritation should be directed at the question -- your answer is spot-on.  "If we ignore all the reasons that this doesn't work & isn't a very good idea, can you in theory do this & make it work?"  Sure, ignore enough reality & all kinds of silly things become "possible."  I think it's entirely fair game to include reminders of how much has to be ignored, with the answer to such a question. +1

Throwing current tech out the window, but not necessarily throwing reality.  It's certainly possible to imagine a compact & powerful fusion generator that doesn't violate the laws of physics.  But I think you would design an electric-powered engine much differently than the fan of a turbojet.

The OP is very clear that they want to know about one specific parameter in isolation from any other technical considerations, it’s strange to think that anyone would take umbrage.

Well, this is the inter of web, so blowing one's top off, for any given reason or lack thereof, is the most likely reaction to any action (Newton's 3rd law, revision 2.0)

There's nothing inherently wrong with focussing on a part and thinking of other ways to do that task.  This is why Rocketlab came to the idea of using electric-powered turbopumps and batteries on their Rutherford engines.  But the flaw comes with figuring out all the interactions with existing systems, and if its a net-win or net-loss.  That might be evened out in manufacturing savings, noise abatement, taxes for dirty emissions, or something still-unconsidered.

@Criggie of course the big difference between the battery-powered electric turbo-pumps on a rocket engine, and electric turbo-pumps for a turbine engine is that one flies for five minutes, and the other flies for five hours.  That makes all the difference.

@RonJohn Totally - and the batteries are just dropped/discarded when exhausted.  But the point was about thinking outside the box, and whether the sum of advantages outweigh the sum of disadvantages.  And some of those are legal or reputation or advertising, hard to calculate in dollar-terms.

@Criggie "or reputation or advertising"  as in "Look how green we are!"

@Criggie as far as thinking outside the box...  airlines have wanted efficient engines for 45 years, and GE, RR & PW have worked hard to make them more efficient.  I'm certain they've all performed engineering studies on alternate methods of powering the compressor fan; it's just so damned practical to steal exhaust gas.

@Criggie The other big difference, apart from the flight time, is that the power to drive the fan of a large turbofan at takeoff thrust is of the order of 50 to 100 MW. The turbo pumps on a rocket engine are not providing all the power the engine is generating.

The problem with the battery is that you have https://avalanchefusion.com/ working on a 100kW fusion reactor that sits on a desk - so it is a feasible long range element that is BETTER than the fuel used for far, from a energy/weight density. My AI calculated the use of 0.15g fuel per 24 hour under full load. In this light, looking at the engine separately to evaluate feasibility makes sense.

Oh wow. Just wow...

"using only the fan and not the exhaust jet" does not make sense.
This is because the exhaust jet powers the fan.  That's what the turbo in "hi-bypass turbofan" means: the exhaust jet spins turbine blades attached to a center shaft, which thus spins turbine blades in the front of the engine.  Those front turbine blades then compress air into the combustion chamber.
Something must power the fan. If you don't "steal" some thrust gas to do it, then some other energy source must, for hours and hours and hours.  Therefore, somewhere in the limited volume and weight requirements of the plane, you must also account for an energy source and motor.
Bottom Line:
Stealing some thrust gas is the most compact and economical method of spinning the fan.

His ideais to power the fan - electric. That is the whole question.

@TomTom the electricity has to come from somewhere.

Multiuple companies working on small size energy generation - i.e. avalance https://avalanchefusion.com/ with a really small 100kW fusion core planned to be done in some more yxears. THAT is a small and low weight long enducance energy source for you.

@TomTom "planned to be done in some more yxears." Hahahaha... vaporware.

like AI was in 2017. They have good investors and are well reviewed. Also, they are one of about a dozen companies readying fusion reactors - the first going supposedly online in 2028 - and their special thing is the focus on small size. You - would consider reusable rockets vaporware - just before it was demonstrated to work. I definitely put them not into vaporware but into active research because of - credibility. That you can - well - check - instead of acting like a brat.

@TomTom it's vaporware until it ships.

would [it] be possible to power the fan in a modern hi-bypass turbofan with an electric motor?

Yes, but it would not be the most efficient way to convert electric energy into thrust. The superior way would be to attach a large, slowly spinning propeller to that engine. Since electric airplanes have a poor power-to-weight ratio once their range becomes practical, the available power should be used most efficiently. Jet engines only come into their own at flight speeds above Mach 0.7 when propellers run into increasingly unsurmountable compressibility problems.

and achieve performance comparable to the kerosene-fueled engine?

Not quite because the fan-motor combination is likely heavier than the jet engine. Small electric motors can weigh as little as 0.1 kg/kW but larger ones struggle to reach even half of that. Siemens demonstrated a GA-sized engine with 5 kW/kg, and scaling this up to airliner power levels would most likely result in no more than 2-3 kW/kg. Even with 95% efficiency, 5% of power will be turned into heat, creating cooling problems for large electric motors.

is the 10% of air that flows through the combustion chamber crucial for the operation of the system?

Yes, it is. Its mixing with fuel of high specific energy and combustion provides the mechanical energy to drive the fan. Its much higher exit speed at the nozzle also lets those 10% contribute a disproportionally higher thrust.

Ignorant to the fact that efficiency for energy is not a factor once you get small fusion reactors generating the fuel - small enough to sit on a desk, 100kW output... https://avalanchefusion.com/tech/ coming in a couple of years (from now, not 2020).

@TomTom Seriously? Fusion has been just around the corner for half a century and will continue to do so for the foreseeable future.

By governments, yes, and they STILL assume ITER will take 30 years. Look at space travel - NOTHING changed until a private company came up. Helion has binding contracts to deliver electricity from a 400MW fusion facility IIRC 2028. They plan to have their first plus-energy-reactor this summer. I know about a dozen private companies that all plan to have reactors running by 2030 and all are financed by large investments. You know what also was around the corner for a century (okish): AI - and AI is here. Robots. Robots are nearly here (look at the demos). Future? NOW.

@TomTom Business plans designed to deceive gullible investors are not infallible prophecies. Get real!

Ah, so now everyone working on something new is looking for gullible investors. Maybe you should talk to a psychiatrist? As I said - plenty of examples making your opinion looking utterly stupid, and here we have a whole industry working on this problem. Believe what you want - reality generally does not care about you.

@TomTom Your cherrypicking makes you look stupid. Let's talk again in 5 years and see who's right!

I believe this has actually been done; please see this video and decide if this meets your theoretical question setting:

This Airbus custom demonstration aircraft flew from England to France using two battery-powered electric ducted fan engines, which could be considered similar to or the same as electrically-powered high-bypass (indeed, 100% bypass) turbofan engines.
At 20:55 the pilot stops the fans prior to getting on the runway and you can see via the front that there are fan blades and a small central core section inside the nacelles. At 21:05 there's a good look from straight aft of the same.
Takeoff is at 24:30 and performance may or may not match your definition of "comparable to a kerosene-fueled engine", but this is reality, not theory, so they were obligated to consider battery and motor limitations.

This answer just triggered me to downvote the question because it is now completely unclear to me whether the OP is simply asking whether electric powered flight was possible, (which I presumed was common knowledge...) or asking about some hybrid combination of thrust from an electrically driven fan augmented by a gas turbine.  The answer is a good one, if the question is about purely electric ducted fans...

I saw the word "theoretical" in the question, and how existing answers all seemed to address the limitations of generating the power to drive the fans in the first place, and I tried instead to address the unanswered aspect of the question: simply whether, in theory, any kind of all-electric-driven (not-really-)turbofan could function as well as current kerosene examples.  The answer is obvious (yes, push air, make plane go) but the questioner also asked about the jet exhaust contributory aspect, which I did not directly answer.  But I gave a practical, extant example, with video.

I'd say a Large Fan moved by an Electric Engine in a Turbofan won't need batteries at all, Hybrid ICE-electric systems may need just an small battery as buffer or emergency safety backup, if the weight of generator needed to supply power to electric engine, it can be turned by the power section shaft of turbine, adds no weight to te current reduction gears or specific turbine to move the fan, and overall efficiency is good, it can be considered, but this needs calculations and wind tunnel/ engine testing. Blessings +

Original question said to ignore issues of battery / power source, so all discussion of efficiency, weight, capacity etc. are IMO not germane to answering the original question.

Long range will not use batteries. https://avalanchefusion.com/tech/ - fusion reactor, 100kW, sitting on a desk, so small. THAT is your power source - using 0.15g fuel per day on 100% load. Practically unlimited range on any small plane.

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Is the published recommendation on when to turn crosswind dangerous?
Citing the FAA from Airplane Flying Handbook

On the departure leg after takeoff, the pilot should continue
climbing straight ahead and, if remaining in the traffic
pattern, commence a turn to the crosswind leg beyond the
departure end of the runway within 300 feet of the traffic
pattern altitude

I am wondering why this would be the procedure, compared to something like standard distances (like a half-mile radius, which is a loose guideline in GA pattern spacing). Or maybe published ground references, like when there is a noise abatement procedure.
Seems like this suggestion causes a hot spot for collision where crosswind meets downwind. I have a real example from a controlled airfield.

tl;dr - A low power plane takes off just before a higher power plane. Both turn crosswind at 700' AGL and downwind at 0.5 miles. Near miss where crosswind meets downwind.
Busy class D airport. 2 Cessna 172's ready for departure. Call the first one 172A. I'm second in 172B.
172A is a 4 passenger 160hp. I'm in a 180hp solo. During run up I hear 172A for straight out departure (this is wrong, I heard incorrectly). I'm a right downwind departure. 172A is cleared for takeoff. I line up and wait. Then my 172B is cleared for takeoff. There is a thick cloud layer 2 miles out at 1000 AGL. I climb to 700' AGL and about 10 seconds into my crosswind turn, 172A appears above near the cloud layer flying directly at me, about 100' above, 500' away. The cloud layer is close ahead (not risking VFR to IMC), and there's a busy class B less than a mile out crosswind, so I round out to downwind early, stay to the right of their track, full power to remain ahead, but pitch level to stay under their track incase they have much higher speed.
I ask traffic on downwind if they see me, and tower comes on screaming that I've cut off downwind traffic, and to increase speed and leave their airspace. They thought I was trying to save myself time, but I thought I was just flying my pattern. Funny thing is, so did the pilot in 172A.
I could have:

Listened better to #1's departure clearance and kept him in sight
Asked for more spacing or clear my crosswind since it was a close departure anyway

ATC could have:

Spaced us more
Call my crosswind

ATC didn't notice me on crosswind until after, but I don't believe ATC was being lazy, they were probably just dealing with inbound traffic or ground. I did later ask them where is a good point to turn crosswind, but they said that's not how it works and I should read the rules. I did, seen above. I met with the other pilot afterwards, and he said the same thing, just flying the standard pattern.
I'm worried that I'm still the idiot here, but I just want to put this out there for consideration and public review in case there is more to learn here.

Lots to unpack, but a note up front: in Class D there is no minimum separation (no set distance) which ATC must ensure between two airborne VFR aircraft. However ATC must issue traffic advisories and safety alerts as necessary, and must ensure proper runway separation; for two departing C172s the minimum is #1 is 3000' down the runway and airborne when #2 begins takeoff roll.

It seems what happened here was your groundtrack was to the right of the other plane on departure. You also may have been climbing faster.  It might have helped to maintain sight of A while climbing if possible.  In spite of being yelled at, your actions may have prevented the situation from getting alot worse.

@RobertDiGiovanni in this case, ground track was mostly straight for both if I recall correctly. It was a few seconds into crosswind at first visual contact. But groundtrack is also a variable in this crosswind-downwind hotspot.

IF 172A is a straight out departure I would not expect them to turn downwind and leave the airport that way.  It seems (1) either I heard the radio incorrectly or (2) 172A did something they did not say they were going to do.

Interesting to think that this may be less likely to happen at a non-controlled airport, where I could've noted the timing of his crosswind/downwind calls (presuming we make them). But with no position calls, ATC only had to look away for a few seconds.

@wbeard52 I just edited for clarification: I just plain misheard their clearance while on run-up. Tower made that clear :)

Bottom line, is you were VFR. YOU are responsible for aircraft separation. Maintain visual contact with the aircraft ahead of you. If you lose sight tell the tower controller, (or if uncontrolled, tell the other aircraft), (172A, this is 172B behind you, crosswind at 700 AGL, I have lost sight...  Say your position and altitude.)

Just for interests sake, the 300ft within TPA mustn't be a global standard. In Australia we turn to the crosswind leg at 500ft. This still has the same underlying risk as you've pointed out but the difference between 500 and 700ft can be close to a minute for a slow climbing aircraft.

compared with standard distances ... or published ground references

As one can see, all of these help maintain separation.
Strictly adhering to "turn crosswind when within 300 feet of pattern altitude" has one obvious flaw, an aircraft with a higher rate of climb will turn sooner.
Plugging in some numbers, one is climbing at 400 feet per minute and the second at 700 feet per minute. With 1/2 mile separation at take off and similar Vy, we can see that the second plane turns almost at the same time as the first.  With no established distance for the cross wind leg, the second plane will turn inside the first.
If the second plane turns a wider radius, and both are trying for the same altitude, they may be on a collision course.
Without references for the crosswind leg being a certain distance from the runway, yes, this could lead to a potentially dangerous situation.
As stated in comments, it is important not to lose sight of the plane ahead of you.  You might have been able to stay outside the A track from the start and climbed with a bit less power to maintain separation. (It is understandable you didn't, thinking they were departing straight out),
The "turn when within 300 feet of pattern altitude" rule is just part of a much bigger safety picture.

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How do jet engine turbines blades rotate in different directions?
I have seen many jet engines' turbine blades rotate in different directions while they look on same shaft.
I wish to understand how they can rotate in different direction.
The picture below explains my question more. Is there a gearbox that changes direction? Or do they use two shafts?
.

No the two shafts are not connected mechanically. They rotate in different directions simply because the turbine blades are in different directions.

Hello eeesa, welcome to aviation.stackexchange.

The image shows a geared turbofan with two concentric shafts.
The outer shaft is connected to the final compressor stages and the first turbine stages.
The inner shaft connects the first compressor stages and the final turbine stages.
Hot air from the combustor drives the first turbine stages (and thus the outer shaft) in a clockwise direction (when viewed from behind in the direction of flight). The vanes of the final turbine stages are the other way around; they rotate the inner shaft (an thus the first compressor stages) in a counter-clockwise direction.
The fan up front is connected through a gearbox to the inner shaft. The gearbox reduces the rotation speed and reverses the direction. Thus the fan rotates clockwise again.
The reason to have multiple shafts and a gear box is that each stage functions optimally at different rotation rates. You want to avoid supersonic speeds as much as possible, since the shockwaves of supersonic blade movement decrease the efficiency. Only the blade tips are allowed to go supersonic.
Therefore, the fan, which has the largest diameter should spin slowest.
The first compressor stages and the final turbine stages have lower diameter, and thus can go much faster.
The final compressor stages and the first turbine stages have even smaller diameter and see very hot air (and thus higher speed of sound) can rotate even faster.
By having two shafts and a gearbox, the design is optimised for every stage. (some designs use three shafts).
Reversing the direction of rotation helps keeping the general airflow aligned with the axis of the engine, which is more efficient than creating a swirling airflow.

is that what happens in military engines too? is this also how the new engines designed?

@eeesa military engines are optimised for different requirements, depending on their intended use. Generally military engines will have different needs for efficiency, reliability, maintainability, maximum power, cruising speeds etc. than the engines used on civil aircraft. The engines on a military transport aircraft are comparable to those on civil airlines. The engines on a fighter jet are totally different, and so are those on cruise missiles.

Flow isn't the primary reason for the high pressure and low pressure parts of the engine rotating in opposite directions Each row of rotating blades is followed by a row of stationary blades so direction of rotation of different sections doesn't matter much for flow. The main reason to reduce the gyroscope effect of the engine, which would otherwise interfere with control inputs (for example exerting a pitch torque when the pilot turns the aircraft in yaw.) To a lesser extent, power changes could exert a roll torque on the aircraft if the different sections didn;t rotate in opposite directions

@LevelRiverSt -- the RR Pegasus in the Harrier is an excellent example of how contrarotating spools helps controllability

@DeltaLima how can I read more on fighter engine exactly? any sources? books? recommendations? pls


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What should the pilots and ATC do if a passenger forgot her baby at the airport?
Recently I saw an article about how a mother forgot her baby at the terminal. After they had already taken-off and climbing, she suddenly realizes that she forgot her baby. According to many sources, she "refused to continue the flight" (to be honest, I don't know what that's supposed to mean; I'll probably post that as another question). The pilots requested permission from ATC and turned back. Now, my question is, are the pilots required to turn back in such a scenario, or can they choose not to turn back? Also, will ATC put the aircraft's priority at the bottom of the incoming traffic list, or it this some kind of urgency flight that ATC must prioritize over normal flights (but below emergency flights, of course)? I'm asking this in a case where there's no safety concerns if they do return, but the pilots and ATC doesn't want the aircraft to turn back and delay others just because of this.
EDIT: The originally linked articles used as reference are not trustworthy and reliable, so I replaced them with a single, more trustworthy source (a The Guardian article). I hope this makes this post more fact-based, clearer, and useful and answerable.

Most of this isn't really covered by aviation regulations and should probably be asked on Law.SE (similar to your recent question). The only on-topic part of this question I see is "can ATC deny the aircraft from returning, or are they generally required to grant permission?"

Also, I'm very very skeptical this is real. All the links go back to the same article, and the supposed "viral video" is no longer on youtube. Lets find out the truth

FYI, I downvoted because (among other reasons...) the answer calls for speculation about what they "should" do without enough detail to form a valid subjective opinion.  (Is it an emergency because they were left in a car in the hot sun in the parking lot?   Or is the baby in the custody of gate agents who are providing adequate care?)  Not really answerable as-is, and not a good fit for this site.

Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on [meta], or in [chat]. Comments continuing discussion may be removed.

What should the pilots and ATC do if a passenger forgot her baby at the airport?

Answering on the principle, as this kind of story is in general at best distorted, as worth totally false. You expect people here to provide fact-checked elements, people here also expect fact-checked questions.
ATC role and reaction to a change of plan
You seem to have a misconception of ATC role. ATC doesn't tell aircraft where to go. They ensure aircraft separation and provide information related to the safety of flight, like other aircraft in the vicinity and weather.
Assuming returning to the origin airport is feasible and this is the decision taken by the crew, ATC will facilitate the change of plan. This means some rerouting will take place using a safe flight path preventing any collisions with other aircraft. ATC will take care of this according to the pilot request. It may be more complex than a simple 180° turn at constant altitude.
Crew decision
It's obvious when considering the impact of a diversion in term of inconvenience for the passengers and the operator, and in term of cost for the operator, the aircraft will not divert. There are airport and operator employees, and authorities who are able to deal with the problem, this doesn't require the mother to be present.
The operator employees on the ground will just call the airport where the baby is and ensure someone will take care of the baby, until the mother or someone else is able to go to the airport.
Authority of the pilot-in-command
As soon as the doors are closed, the PIC has a large authority on the passengers in order to ensure the security of the flight. The fact the mother is not accepting to continue the flight is an element which has little impact on the PIC decision, motivated only by the baby own situation. From SKYbrary:

ICAO Annex 2, par. 2.3.1, specifically empowers the PIC to override any other regulation in an emergency, and to take the safest course of action at his/her sole discretion. This provision mirrors the authority given to the captains of ships at sea, with similar justifications. It essentially gives the PIC the final authority in any situation involving the safety of a flight, irrespective of any other law or regulation.

Aeronautical rules inherit from marine rule. Nobody imagines a ship would return to the port of origin after a couple of days of navigation because a baby has been forgotten.

Hmm. (point 1 is just pedantic: ATC can tell pilots not to flight on some areas). Point 2: authority: it is ok, but PIC having authority doesn't mean PIC is above law or PIC has full discretion. It just means that PIC has full control, but later laws/companies can intervene. Point 3- If a stowaway is found, ship companies usually requires ships to return if they are around 2 to 4 days away: it can be much cheaper. If a baby is found on a ship, they will return also much later. The contrary case it is just seldom: ships with long tracks without ports will not allows baby anyway.

@GiacomoCatenazzi: Yes, but formally the objectives of ATC are only to: "prevent collisions between aircraft, expedite and maintain an orderly flow of air traffic,  provide advice and information useful for the safe and efficient conduct of flights, notify appropriate organizations regarding aircraft in need of search and rescue aid, and assist such organizations as required." (Annex 11). My point was there is nothing like "prohibiting to return", or  "air police" in ATC objectives, only clearance, advice and information.

The short answer is that Federal Aviation Regulations do not require either Air Traffic Controllers or Pilots in Command to take responsibility for the consequences of passenger negligence, or the negative consequences of passenger inactions, on the ground either before or after flight.
This may sound a little cold and heartless in the case of an infant, but where would you draw the line?  Could a passenger declare that they are unwilling to continue because they forgot to turn the coffeepot off back home?  Or because their traveling companion was in the restroom during boarding and missed the flight?
Now, there may be a way to force the issue as a passenger if you become unruly enough, but there will be consequences...
The best course of action involves judgment, not regulation.  A good option for the crew might have been to relay the situation to dispatch to ensure the infant is well cared for, continue the flight, and let the passenger deal with the logistics of ensuring longer term care and reuniting with their baby later.

I'm wide open to feedback from downvoters...


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Is it possible for one wing to stall due to icing while the other wing doesn't ice?
Is it possible for one wing to freeze with ice completely to the point of stalling, while the other wing is fine? Let's say the pilots made a mistake by forgetting to turn on anti-icing systems. Is there any possibility of this happening, and if so what kind of phenomenon would cause such a weird situation? I'll appreciate any explanation. Thanks in advance.

Not a duplicate of why would one wing stall before the other, since I'm asking specifically about icing here, not one-wing stalling.

It's not impossible I'd say, what about flying on the edge of the cloud with localized icing? if your plane is big enough you could have that kind of distance. And as you mentioned the icing system failing on just one wing is enough to do this too.

I have personal experience with a crisis on the CRJ200 program where the outer section of wing was not getting hot air, and the only warnings were random bleed leak warnings that were No Fault Found during ground ops testing. Finally someone investigated inside the LE and found the piccolo distribution tubes were rupturing at mid span. Airplanes went for extended periods without outboard wing anti-ice. How nobody crashed on an approach in icing conditions was a miracle.

@MichaelHall whether or not someone believes something is possible/true bears absolutely zero relevance when it comes to asking about it!  In fact,  the whole scientific research methodology is based upon you believing something and trying to prove it wrong or vice versa. Look up null hypothesis.

Point taken @MichaelHall but this is far from a question about unicorns. Any reasonable adult should understand that inquiring about something that is contrary to ones beliefs is very much valid, even more so than seeking affirmation to own ideas. Hence the downvoters are tragically wrong

Curiosity is enough!

@J.J., I understand and agree, but a voter's personal opinion on the quality of any given question is enough too!  ;)

Hey, what's with the 7 downvotes? Please tell me how to improve this post so that other users can benefit from it. I wish to contribute to this site more, so please tell me how to make my post better. I'll appreciate any suggestions or edits. Thanks in advance!

It's effectively impossible for one wing to be in meteorological icing conditions while the other isn't (at least for any meaningful length of time), but there are certainly several ways for an aircraft to wind up with one wing affected by ice while the other is not.
First and most likely, it's entirely possible for the anti-icing system on one wing to fail while the other is still working normally, which would potentially lead to an asymmetric ice buildup. There are many different types of anti-icing and de-icing systems*, so the specific failure modes depend on the details of the airplane in question. But in any case, each wing's ice management is a physically separate system, which means one can fail while the other doesn't. In a case like that, I'd expect a notification to show up on whatever error reporting system the airplane has -- ECAM or EICAS or just a dashboard light to indicate that something is wrong with the anti-icing without detailing what the problem is. Again, this depends on the details of the airplane in question and the details of what has broken. It's possible the failure may not be reported adequately to the pilots, depending on what exactly has happened.
Second, it's possible for deicing boots to make the ice crack off of one wing while the other wing doesn't shed its ice load fully. That could possibly cause one wing to have different flight characteristics from the other, making one wing more prone to stalling than the other, though I would expect the difference to be relatively minor and only impact the airplane as a whole if it was already trying to operate close to the stall speed. One would hope that in this case, the pilots would get a stall horn when the more stall-prone wing got close to its new, higher stall speed and take appropriate corrective actions, even if they don't understand why they're getting an early stall warning.
And third, natural ice buildup isn't totally symmetrical, so even in the absence of any deicing system activation, you can have the same situation as the boots happening naturally. The ice on one wing might form in such a way that most of it sloughs off under the usual aerodynamic forces and wing-flexing, while the other wing's buildup just happens to form in a way that doesn't lead to the ice departing on its own.
*There's a distinction between anti-icing systems like leading edge heat, which prevent ice from forming, versus de-icing systems like inflatable boots, which help remove ice after it has formed but don't prevent it from forming in the first place.

I dont think it is realistic that the airframe would accumulate ice only on one side. However, it is very possible that stalling due icing occurs first on one side while the other wing is still below the critical angle of attack. Actually this is most likely scenario to happen as heavy icing affects the airflow in very unpredictable ways.
It is also possible that when the wings are being de-iced the ice breaks unevenly. This is specially problem with so called de-icing boots which remove ice mechanically by inflating “airbags” that, when expanding, break the ice. If used too early there is not enough ice to crack but instead ice may form a cavity around the leading edge where the de-icing boot can expand without ever even touching the layer of ice. If this happens only on one side, the wings can have a very uneven distribution of ice. However, this is unlikely with modern turboprop aircraft which have automated de-icing systems and more sophisticated de-icing boot design. Jets use hot air to heat the leading edges and are not affected by this phenomenon.
The other scenario is that aircraft doesn’t have de-icing systems (or they are not used) and ice breaks away unevenly after the aircraft leaves the icing conditions. This can leave wings temporarily (which may be enough to cause very serious problems) unevenly iced.

Is asymmetric icing possible (and perhaps even realistic) when the sun is abeam and low in the sky, putting one wing in the shade?

@TobySpeight Maybe while parked, but not at flight speeds when the "wind" overwhelms the miniscule heating from a near-horizon sun onto a nearly horizontal surface.

'Technically' Yes, it is completely possible for one wing to freeze with ice and stall (somewhat like a wing drop) whilst the other wing is left without a hint of ice in a situation possibly where the heating system for our frozen wing hasn't been activated (and our 'unfrozen' wing's heating systems have been) due to pilot error and such and additionally, you can also have a system
malfunction.


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Is there ever a manned aircraft that was successfully flown faster than the Concorde?
I've always been interested in supersonic airliners, like the Concorde. However, my question is, is Concorde really the fastest manned aircraft in the world? Had there ever been another manned aircraft successfully flown faster than the Concorde? Thanks for any answer!

Concorde was one of the fastest airliner not because it was incredibly fast, but because it was incredibly impractical, so much that we have completely given up on the idea of supersonic passenger airplanes till today. It wasn't even the fastest airplane even back in the 60s.

Before we get into a series of fastest/highest/ slowest manned/ unmanned questions, take note of the requirement to do some basic research before posting here. Wikipedia has a page dedicated to air speed records, and links to other aviation related records. FWIW, the record for fastest manned controlled flight is held by Space Shuttle Columbia at around Mach 25.

If you hover over the downvote button, one of the suggested reasons for downvoting is "lack of research". Now, I didn't downvote, but I can understand why someone would downvote a question for "lack of research" when the answer can be found using any random web search engine in less than 5 seconds.

I am curious as to why you think the Concorde is the fastest manned aircraft, or even one of the fastest?  Can you point to a website that gave you that impression?

@JörgWMittag So, can I improve anything? Because I can't delete this question anymore, so I would like to take some actions to prevent further downvoting. Thanks for your explanation, and I would very appreciate any suggestions for improvement on this post.

@MikeSowsun Well, there are many places that state Concorde is at least one of the fastest aircraft in the world. Various articles, websites, TV channels, etc... , to name a few: worlds fastest airliner, article on Concorde from the Guardian, etc.

@AircraftEnthusiast007 - There are a LOT of military aircraft that are faster than the Concorde. Going all the way back to the 1960s, and the F-106. Trivial research would discover this.

The fastest powered, crewed aircraft was the North American Aviation X-15A-2 Number 2 which reached an airspeed of 7 274 km/h corresponding to Mach 6.70 on Flight 188.
Ironically, the fastest powered, crewed aircraft is not also the fastest crewed aircraft in general. The fastest crewed aircraft is actually a glider: the Space Transportation System Orbiter OV-102 reached about 28 000 km/h corresponding to about Mach 25 on STS-2.
There are some unofficial claims that the Buran reached speeds slightly faster than that. IFF that were true, it would make Buran the fastest crew-capable aircraft: it was designed for a crew of 8–10, but its only orbital test flight was performed uncrewed, then the program was shut down before any crewed flights happened.


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What is the largest possible weight a parachute can support? Can parachutes support extremely heavy objects like elephants?
I know that some parachutes can be attached to objects heavier than humans, like weapons, emergency supplies, etc. I also know that some military specially trained dogs are sometimes dropped by parachute in a military operation, attached to their handler or trainer. However, can parachutes support huge, extremely heavy objects, or animals like elephants? Basically, my question is:

Had there ever been an occasion where an object weighing a few tons (or elephants, which usually weigh more than 4 tons) parachuted out of aircraft? I'm asking regardless of it's a military operation or not.
If not, is this physically possible given today's largest and most durable parachutes? Can today's largest parachutes be able to withstand the weight of something similar to an elephant and still function normally/properly (i.e., slow the rate of decent)? If not, is it possible to design a parachute to support massive cargo/occupants?

I know the elephants part is a rather crazy and non-sensical concept, and I know that it would not be reasonable and actually practical to do this, but I'm asking for the conceptualized version of things. Basically, my main question is, what is the greatest weight current parachutes can support? What is the greatest possible weight a parachute can physically withstand?
I'll greatly appreciate any insights into this. Looking forward for an explanation! Thanks in advance!

I feel these questions are getting perilously close to off-topic. There is a clear list in the [help/on-topic] that defines the scope of the site. Increasingly unlikely questions about what can be loaded onto or thrown out of an aircraft do not feature. Before posting a question about  <insert object/animal here> ask yourself what purpose is being served. Why exactly would one parachute an elephant out of an aeroplane?

@CatchAsCatchCan What do you mean by "Increasingly unlikely questions about what can be loaded onto or thrown out of an aircraft do not feature"? This is a question about aviation, and not opinion based because I asked if this has happened before and if this is physically possible.

@CatchAsCatchCan I made it pretty clear that I'm not asking if this is good/bad or if it is fanatically possible. I'm asking if it is possible according to physics and aerodynamics. So I don't see why this is off-topic. If you think this is not possible according to physics, then just post an answer telling me so, rather than downvoting my post.

VtC: This question is not about aviation. It's about parachutes. It's about cargo. D/V: I have D/V as I don't believe it is a useful question AND you show no initial research, both of these are reasons for down voting. These are my opinion and I am as entitled to it as you are to disagree.

As for the question if an elephant has ever been parachuted out of a plane I cannot say, but for the technical part:
Yes, it would be technically possible to parachute an elephant out of a plane. As an example of parachute performance, the SpaceX Dragon 2 re-entry vehicle has a "landing weight" of approximately 7000kg, while an adult African elephant can weight up to 6500kg.
While this would be technically possible, I seriously doubt the sensibility of such a stunt. It would be extremely stressfull to the animal itself, and the operation would require sedation. To safely land the animal, some kind of a crate and/or a cushioning device would be necessary, since the elephant as a massive creature is not "designed" to handle impact loads well.

The Space Shuttle SRBs had an empty mass of about 90000kg and landed under three parachutes, each of which was rated for about 88000kg. A single one of those parachutes could carry 13 elephants. The "how" is not a problem, the issue, as you say in your last paragraph, is "why the frack"?


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Could an aircraft possibly achieve a vertical nose dive at 90 degrees without breaking up?
I've been wondering, is it possible for an aircraft to remain intact even if it is in a vertical nose dive? Is this physically, structurally and aerodynamically possible?
In March 21, 2022, China Eastern Airlines Flight 5735 crashed at high speed into the ground, killing all 132 passengers and crew on board. The investigation isn't complete yet, but a security camera on the ground captured a video moments before the crash and revealed a shocking phenomenon: the plane was almost on a vertical nose dive at extremely high speed. However, I've been wondering, is this really possible? Maybe the angle the camera was positioned with respect to the aircraft somehow made the aircraft look like it was on a vertical nose-dive? Because, wouldn't the aircraft be torn apart if it were in such an attitude? Data suggests that the aircraft was descending at speeds of 640 km/h (~0.5 Mach) at first and then reached  or even exceeded 1126 km/h (~0.94 Mach) in the final moments before impact (link). According to my research and knowledge, any aircraft would break apart mid-air if

An explosion that damages the fuselage, no matter how small. This can be inside of the aircraft (for example, a bomb or a fuel explosion), or outside of the aircraft (for example, missiles, think MH 17).
Rapid depressurization at a high altitude, which almost always causes structural failure.
An aircraft not designed to fly at sonic or supersonic speeds (like the supersonic jetliner Concorde, with a maximum speed of Mach 2.04) but somehow is flying at that speed.
Extremely violent movement and turns or severe wind and turbulence combined with a weird attitude (or AOA) and high speed.

Flight 5735 seems to meet the last two conditions. So, my question is, is it really possible that Flight 5735 was in a vertical nose dive towards the ground at nearly Mach 1 and didn't experience an in-flight breakup? Or, was it just not possible, or that the plane did break up in-flight (unlikely, however, because the video shows the aircraft intact). Most importantly, is it generally possible for an aircraft to dive nose-down vertically at high speed (which will always happen because of the attitude and AOA) and not explode/break up in-flight? Glad if anyone can give me some insights into this question. Thanks in advance!

Note: I'm not looking for speculation or explanation/investigation on how/why China Eastern Airlines Flight 5735 crashed. I'll leave that for the officials. I just used it as an recent and extreme example here. Rather, I'm looking for a general answer for the structure and performance of aircraft with bases in physics and aerodynamics.

The essence of your question seems to be, can an airliner (or an aircraft in general) undergo a sustained vertical dive without breaking up, i.e. without experiencing substantial structural failure.  I would suggest modifying the question to feature the "without breaking up" part more prominently in the title, and in the opening few sentences.

@quietflyer Thanks a lot for the advice, I've edited my post to reflect the change. Can you please check it out? Do you think my edited question convey my intention more clearly?

Any reasons, downvoter? Please leave a comment telling me how I can improve. Also, if you had downvoted me because of the reason @quietflyer pointed out, I've already edited my post to express my intentions more clearly, so please check it out. If you think my edit improves the post and the new edited post is good enough, then please upvote me back so others browsing the Questions list will view my question and possibly answer. Thanks in advance for the attention.

Normal transport aircraft (FAR Part 25 certification) have structural design cruise speed called VC.
These aircraft must demonstrate that they can achieve a maximum dive speed VD.  This speed must satisfy:
VC < 0.8 VD
If the speeds are limited by Mach number (instead of equivalent airspeed), you can use the same ratio for Mach number.  There are other ways to demonstrate compliance, but this ratio is the simple one.
The published maximum operating speed VMO/MMO for the 737NG is 340 KCAS / 0.82.
VMO can be chosen up to VC.  So, if VMO = VC, then VD must be 425 / M 1.025.
In flight test, the aircraft will be placed into a dive until it reaches VD and is then safely recovered.  Typically, the active limit in a dive is flutter.
In cases where the aircraft is too draggy to reach VD/MD (even in a powered dive), then the more complex rules come in for establishing what VD / MD must be.
For example, the aircraft starts at VC, is placed into a 7.5 degree dive for 20 seconds, and then recovered at 1.5g.  The maximum speed achieved in this 20 second dive can be use as VDF.  This test would be repeated at multiple altitudes etc.
VDF is the flight test demonstrated dive speed -- this is the highest speed achieved in the flight test of an aircraft.
So, back to your question....
When you place an aircraft into a dive, it will accelerate.  A subsonic aircraft will experience a rapid rise in drag as it approaches Mach 1.  This means that these aircraft will reach a terminal velocity in a dive (even power on) that is not terribly higher than their cruise speed.
Transport aircraft must demonstrate that they are structurally sound in a dive.  This includes avoidance of flutter in the dive.
It is possible that a transport aircraft could be placed into a dive and would reach terminal velocity before it would reach structural failure.

Thanks for the answer. So you mean that aircraft actually can not exceed its operational speed too much, and thus won't break up.

Re "Transport aircraft must demonstrate that they are structurally sound in a dive. This includes avoidance of flutter in the dive."  -- surely not a vertical dive, which was the subject of the question.  Re "It is possible that a transport aircraft could be placed into a dive and would reach terminal velocity before it would reach structural failure."  --  I find this hard to believe.  But, then, you learn something new every day--

@AircraftEnthusiast007 Correct.  The transonic drag rise of these aircraft is quite dramatic.  Before Yeager and the X1, some doubted if it was possible to break the "Sound Barrier".

@quietflyer The orientation of the dive does not matter in the flight test for demonstrating VD.  It does of course matter for establishing the terminal velocity.  Look at transonic drag rise, it presents a near vertical wall.  The increase of drag with speed in this region is tremendous.

@RobMcDonald Thanks for the explanation! I do have a question though: does the attitude of the aircraft really don't matter? I figured that if for example, the wind is blowing horizontally, is it true that the stress on the structure of the aircraft is equal for one that is traveling straight and one is descending straight down in a vertical nose-dive?

@AircraftEnthusiast007 My point was that the regulations don't say anything about the aircraft's attitude when demonstrating VD.  You set VC, compute VD, then conduct flight tests to demonstrate VDF.  The attitude does not matter.  When conducting structural analysis, engineers typically consider the speed and the load factor (g's), but don't worry about the attitude.  Aircraft are naturally very strong in the nose-to-tail direction.

I assume your question is to exclude fighter jets and unlimited category aircraft which by design certainly can do pretty much everything, the physiology of the pilot being the limiting factor.
First of all as to your research on the reasons that would lead into in flight break up:
There is at least one incident where an airliner has sustained a hit from a missile, but was able to land, the Korean Airline flight 902.
Rapid depressurization as such does not necessarily cause in flight break ups. There are several cases of rapid, even "explosive" depressurization with limited or even no damage to the airframe.
There are documentes cases in which an airliner has lost parts of its fuselage and still managed to remain controllable.

United Airlines flight 811 lost a cargo door and surrounding structure at high altitude, yet it landed safely.
Aloha Airlines flight 243 lost a substantial part of its fuselage during flight, but incredibly enough the crew managed remain in control and land the plane

As for the question itself whether or not it is possible for an aircraft to dive vertically and not break up I would say yes, it is, at least for a short period of time. An example of this would be the China Airlines flight 006. Due to engine failure and improper control inputs the aircraft tumbled into a dive losing 30000 feet of altitude in 2.5 minutes. The flight data recorder only worked periodically during the dive, and there is no certainty that an actual fully vertical dive was present at any time, but when it comes to stressing the airframe this case is extreme: loadings exceeded 5 g's and there was severe vibrations due to high speeds.


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Has an aircraft ever crashed due to it exceeding its ceiling altitude?
All aircraft today have a ceiling altitude that they cannot fly beyond. The most common range is around 35,000~42,000 feet for large, wide-bodied commercial jets. However, my question is: had there been any cases where a commercial jet somehow flew beyond the service altitude and crashed as a result? In other words, are there any occasions where a commercial jet crashed or experienced problems because they went too high (eg, 42,000 feet or higher)?

The max altitude for most airliners is generally set because the engines are not able to produce enough thrust to go any higher, not because the airplane wouldn't structurally be able to survive at higher altitudes. So you can't just accidentally go higher than the max altitude, since the engines are not physically able to produce enough thrust

Technically, the four de Havilland Comet I crashes in 1954 come close. They flew within their certified altitude range but cabin pressure cycling led to catastrophic failure.

@PeterKämpf Hi, thanks for your answer. I checked it out and found it to be just what I'm looking for. Again, thanks for the information! I suggest you post it as an answer so I can accept it.

I did not downvote but it's reasonable to conclude something thinks it doesn't show research effort, it's unclear, and/or it's unhelpful. It seems to show a lack of understanding of the basics of flight, and it's a little confusing that the word "exploding" appears in the title and isn't clarified in the question itself. Also an answer would be more meaningful to you than to the general user base. So it could be argued this question meets all three criteria for a downvote. Normally I don't like to comment when I downvote because it seems to invite more arguments instead of edits.

If you'd asked for aircraft rather than aeroplanes, then almost every weather balloon launched explodes after flying too high

A plane doesn't simply "explode" on high altitude because the hull and the pressurization systems are built with safety measures.
However, there are safety problems with flying beyond the specified altitudes. The lift is reduced and the engines produce less thrust.
In a wider sense, West Caribbean Airways Flight 708 can serve as an example of a crash due to "flying too high":
In their calculation of the maximum altitude the pilots didn't account for the anti-icing system that draws power from the engines and thus reduces the maximum safe altitude. Because of this mistake, the plane flew beyond the theoretical safe altitude on autopilot and gradually lost speed until it entered a stall, from which the pilots never recovered, partly because they incorrectly assumed that a flame-out of both engines was the cause of the sudden drop.
Of course, in this case the unsafe altitude alone didn't make the plane crash, but there were other contributing factors like less than perfect behaviour of both the auto pilot (should not silently let the plane enter a stall) and the human pilots (failed to recover from the stall). Nonetheless, the cradle of all the trouble lied in the unsafe altitude.

Yes, you are right. Remember that (1) is not a problem in itself, it just slightly reduces thrust and therefore the maximum safe altitude. 31000 feet would have been a perfectly safe altitude with the de-icing system on. However, at 33000 feet there was not enough thrust in combination with the de-icing.

Yeah, if the captain had not mistaken stalling for engine flameout, the aircraft would've been able to avoid disaster. Also, I reasoned the autopilot on the aircraft was poorly designed as it did not give a clear warning of the impending stall, and didn't even report the condition after the aircraft actually did stalled. The only thing it did was to increase the AOA to 5.8°, which worsened the situation.

Airplanes are dynamically pressurized, not sealed.   If the plane could be lifted too high, it would not explode (or otherwise fail catastrophically), but the interior cabin pressure would drop.   Besides the normal operation of the pressurization system, there are safety valves that would open if the pressurization difference were too great, preventing something like a fuselage failure due to excess pressure.
Above the absolute ceiling an aircraft can generally not maintain altitude, and attempting to do so can cause problems.   See coffin corner.   I'm not aware of a commercial aircraft crash that was due to loss of control near the ceiling.
One aircrew did attempt to fly a plane "too high" and crashed it.   But the altitude certainly didn't cause the crash.

Hi, thanks for the answer. I read the article on Wikipedia about the crash of Pinnacle Airlines Flight 3701 through the link you provided. This accident is similar to what I'm looking for, but the main stated cause of the accident is not flying too high.

@AircraftEnthusiast007 There were a lot of poor decisions made one after another during that flight. But what started the whole accident chain was their choice to climb that high too quickly. This left them unable to maintain airspeed at that altitude and meant that a stall was inevitable if they tried.

The coffin corner kills either by stalling you, or supersonic airflow.  While it's triggered by going too high it isn't height related per se, it's just the point where your max safe speed and minimum safe speed cross, forcing you to break one of them.  Most aircraft will run out of power before reaching the coffin corner.

That is an incredibly strange story about the Pinnacle Airlines flight!!  How bizarre. Thanks for the link

Would any of the aircraft designs where an excessive angle of attack can disrupt control flow over the control surfaces, in such a fashion as to prevent recovery, be prone to enter that kind of stall at the service ceiling?

It might also be worth pointing out that at 35,000 feet, 80% of the surface-level air pressure is gone. You're already most of the way to a full vacuum. So the force on the aircraft due to pressure doesn't get that much stronger as you keep going up.

One of the most well-known failures in aviation history, that of a number of De-Havilland Comets, was because of the pressurized cabin combined with metal fatigue. So we could say that for its design it was flying too high. (Although it was not known at the time so it did not exceed any officially assigned maximum value.)

You could also describe some other loss of pressurisation events as 'exploded flying too high' https://en.wikipedia.org/wiki/Aloha_Airlines_Flight_243 though as per the comet they were not flying above design height, but suffering an issue that rendered the design assumptions invalid.

they didn't however explode.

There was a twin jet commuter plane that crashed because the pilots flew it up to 41,000 feet while mismanaging their autopilot, their AoA, and their engine power settings. This led to a both-engines flameout in which they stalled the wing and from which they were unable to airstart the engines because of mismanagement of their descent speed. Both were killed in the resulting crash.

Hi there, thanks for your answer, but can you include the flight number and some links to other resources? Also, I'm wondering if you're talking about Pinnacle Airlines Flight 3701, which the first answer already mentioned. So, are you talking about that case?

yes that's right, I am. -NN

The Comet was imfamous for illustrating the risks of metal fatigue, and every plane since had been a lot safer in that regard.
However there have still been more recent cases of abrupt structural failures during the pressurization (climb) phase of flight.
In some notable cases, including JAL123, tail strikes were incorrectly repaired, eventually leading - sometimes a decade or more later - to abrupt aft pressure bulkhead failures.
The resulting explosive decompression tore the tail section off the fuselage, which in turn led to an uncontrolled descent, and a catastrophic impact with the terrain. (These have been covered by multiple commentators, including the Mentour Pilot and Qnxr YouTube channels.)

no explosions though, so don't meet the stated criteria.

JAL123 is a much better example than the Comet(s) since it might not have crashed if it had stayed below a specific altitude. The Comets are a poor example since the fatigue was caused by the pressurisation cycle, not by a specific height, and would undoubtedly have crashed eventually, even if restricted to a low ceiling.

@jwenting how do you define "explosion" so that it does not include "ripped the tail off"? Given that the OP was talking about "flying too high", they clearly had in mind "popped balloon" rather than "detonated fuel".

An explosion would be the pressure cabin breaking apart due to overpressure, NOT metal fatigue causing a structural member or piece of skin to break.

@jwenting  For the Comets, it is debatable, but for the JAL123 failure, the result of the fatigue was indeed a (mechanical) explosion of the pressure cabin; very, very similar to popping a balloon.

@jwenting yes exactly. With the bulkhead weakened by a fairly small crack, the internal atmospheric pressure tore the aircraft open in a fraction of a second. The rear bulkhead is a critical part of the pressure cabin, and the high-speed debris from the explosion destroyed the rudder and elevator control systems. The cockpit voice recorder recorded a loud "bang" shortly before the plane started an uncontrolled dive.


text:
What do pilots do if they *really* need to go to the toilet in critical parts of the flight?
This question is a silly one, but I would like to ask anyway. What will pilots do if one of them really must use the bathroom in critical parts of the flight like when landing? I mean, like really need to go, probably because of some spoiled food or a minor medical condition. Are there any official guidelines (probably not) or typical actions to reduce the risks of flying with only one pilot? Any thought on this? I'll appreciate any answer, preferably from a pilot's first-hand experience.

Is this *really* about aviation?!

@Jamiec Yeah, why not? And why the downvotes?

So many possible duplicates. Hopefully someone will pick one and VtC as dupe.

@AircraftEnthusiast007 I lean towards agreeing with you, we get so much aerodynamics and engineering stuff on aviation stack exchange, so why not something like this.

Minor medical condition: the pilot should be prepared: diaper or extra wears in bag, Else I would not consider it minor (for a pilot).

Are you sure? And do you thing you will embarrassed by medical conditions? Maybe the first week. There are various medical conditions which requires it, and you probably never noticed (but probably it requires some medical check). If it happens just one every few years is something, else ... really you should plan. Planing is an important bahaviour which should be in nature in every commercial pilots.

Any comment from downvoters? Please tell me where I can improve my post. Thanks in advance.

Please, anyone tell me where I can improve? If there's anything I can improve, please tell me. Thanks.

Pilots typically plan ahead to avoid needing a toilet break during critical phases of flight, such as landing or takeoff. For example, I fly the Boeing 737, and aim to use the bathroom around 300 nautical miles before the top of descent (If I predict I will need to use it before landing). This timing is usually about 50 minutes to an hour before landing, giving me ample time to return to the flight deck, conduct descent and approach planning, and complete any necessary briefings.
However, in the event of an urgent need due to something like spoiled food or a minor medical condition, the situation could potentially escalate to what is considered "Pilot Incapacitation." Pilot Incapacitation is a scenario covered by various procedures and drills, both for the flight crew and cabin crew. Each airline has its own set of protocols for handling such situations. If the incapacitation is severe enough that the flight effectively becomes a single-pilot operation, the remaining pilot would most likely declare an emergency ("Mayday") over the radio to prioritise the flight's safe completion.

Some specifics from your question:

What will pilots do if one of them really must use the bathroom in critical
parts of the flight like when landing?

If they sense a bad bout of something, they may try and let the other pilot know before commencing the approach, then use the toilet. If they have commenced the approach, they will probably break the approach off, discontinue, hold somewhere, so that pilot can use the toilet. If they still feel bad, then it will go down the pilot incapacitation procedures.

Are there any official guidelines (probably not) or typical actions to
reduce the risks of flying with only one pilot?

During our Human Performance and Limitations pilot theory studies, we are taught that during long haul operations, pilots will eat different meals to avoid both pilots getting the same food poisoning. I am not a long haul pilot so can't speak to this, but there is discussion about this in various places: simple flying and other questions. Most short haul pilots (Europe at least) will not be provided any food on the aircraft by the company.
The Company's Operations Manual 1 would typically state that pilots must report in a "fit" state for duty, which mitigates the risk of flying with a pre-existing medical illness.
1: Citing regulations like: CAT.GEN.MPA.100, 170, 175 (EASA) or 14 CFR 91.3, 121.533, 121.580, 91.13 (FAA)

Thanks for your great answer! A pilot stuck in the bathroom isn't what I would normally call an "incapacitation", though. Still, your answer is helpful and informative. I upvoted it, BTW.

@AircraftEnthusiast007 "Pilot incapacitation refers to the inability of a pilot to perform their flight duties due to a sudden medical or psychological condition, which may render them either partially or completely unfit to operate the aircraft." EASA, Safety Information Bulletin (SIB) No. 2020-03.

OK, thanks for the clarification. I mistaken "incapacitation" for "passing-out". Sorry about that.

Oh, sorry for not accepting your answer earlier. I was busy with other things and other posts the last few days, and originally I was waiting for possibly more answers. I always wait a few days before accepting an answer, even if it's superb. That's the case here. Again, thanks for the great answer!


text:
Is it acceptable to abbreviate “Foxtrot” to “Fox”?
My instrument instructor pointed out that I’ve been saying “Four Seven Fox” instead of “Four Seven Foxtrot”. I realize I’ve been doing this for years now, but this is the first I’ve ever been called out on it. I don’t usually fly a plane with an “F” in the tail number but now my regular club plane does end in Foxtrot so I want to fix the habit. For how common it is though, I found surprisingly little commentary in the (FAA) FAR/AIM or on here. I hear it all the time on radio, but that doesn’t tell me if ATC finds it bothersome or how bad it is that I've said this however many times.
Is abbreviating “Foxtrot” to “Fox” acceptable for ATC comms?

Just a little colour commentary. When I worked at IBM, we used the phonetics Able, Baker, Charlie, Dog, Easy, Fox. This is the old US Army/Navy phonetic alphabet from WWII. Are there a lot of older pilots in your area? Or old IBMers?

Adding to the color commentary FWIW, it used to be Fox until 1956:  https://blog.privatefly.com/history-of-the-nato-phonetic-alphabet.  And IMHO there are more annoying phonetic faux pas than this, such as responding to a radio check with "Lima Charlie"...

"Fox" sounds too much like "Cox", might make someone think you are flying one of these https://www.ebay.com/b/COX-Control-Line-Freeflight-Models-Kits/34054/bn_1930286

@MichaelHall what's wrong with Lima Charlie?

@ghostly_s, India Uniform, Alpha Delta Alpha Charlie.

@MichaelHall sorry but that means nothing to me.

@ghostly_s You are not making any improvements by answering "LC" in the form of "Lima Charlie" instead of saying "Loud and Clear" (or 5-by-5 or whichever you pick) - is what my interpretation of it is... Then it is just unnecessary obfuscation for no abbrevial purpose.

Exactly, plus it adds a syllable.  It's unnecessary, and doesn't add clarity.  Because if you espouse a Lima Charlie response, why stop there?  Tower queries whether you plan a touch and go or full stop... why not respond Tango Golf, or Foxtrot Sierra?

Imagine in a bad audio situation all you normally hear and are focused on is "trott"

@ghostly_s, your comment, and the 6 upvotes, illustrates my point that substituting the phonetic alphabet for plain language terms is unnecessary, and doesn't add clarity.  It isn't standard phraseology, and isn't even proper use of the phonetic alphabet.  Because if the receiver didn't understand "loud and clear" you would spell out Lima, Oscar, Uniform Delta... etc.

I just happened to pass the phonetic alphabet pronunciation chart in the Instrument Flying Handbook. There were a few mild surprises- "OSS-CAH", "VIK-TAH", "FOW-ER", "FIFE" and everyone's favorites "TREE" and "NINER" https://www.faa.gov/sites/faa.gov/files/regulations_policies/handbooks_manuals/aviation/FAA-H-8083-15B.pdf#page=55

Use the standard terminology to reduce the chances of ambiguity. Syllables are cheap.

Simply put: no, it is not ok to abbreviate the phonetic alphabet.
The reason the phonetic alphabet is used in aviation is to make communications as clear as possible with minimum chances for misinterpretations.
The terms for each alphabet are chosen such that letters that otherwise would sound similar, especially over poor connection, are pronounced in a way that leaves pretty much zero chance of getting them wrong.
The basic mechanism that humans use to "decipher" vocal communications is distinguishing the pronounciation of wovels and consonats, and observing the  actual length of the word. To properly catch a word these need to match to a certain extent, and over poor transmission they can compensate for each other. Words of medium length offer the best combination of the properties to ensure correct delivery and this is what the ICAO phonetic alphabet aims for. It must be noted, however, that the ICAO alphabet has two somewhat peculiar intances in this regard: Mike for M and Golf for G. These are very short words with no variation in them.
Although one could come up with similar terms off the top of their head, it is important that the system remains strictly defined. This will enable quicker learning of the system, and makes it extremely reliable everywhere. The system is used uniformly all around the world. There are, of course, national versions for different languages, but for aviation English it is always the same.
Please keep in mind that there are rules for numerals too: Three is pronounced "Tree", four is pronounced Fow-er, five is pronounced "Fife" and nine is pronounce "Niner".
This blog post offers a nice brief history of the phonetic alphabet:
privatefly.com - history of the nato phonetic alphabet

"Alphapet"... Fox... I see what you did there.

I find the inclusion of "Foxtrot" and "Oscar" a bit curious, since they have the same vowel pattern as well as similar middle consonants.  Still, shortening "Foxtrot" to "Fox" could result in it being heard as "Fah[static]" which could be misinterpreted as "four".

@supercat in which accent are foxtrot ("o, o") and oscar  ("o, u") similar? I know that accents shift vowels all OVER the place, so I'm not surprised, though.

Isn't "four" pronounced "fower" as well?

@DewiMorgan Phonetically, Both can commonly use /ɑ/ or the "ah" sound. Foxtrot being F"ah"ks-tr"ah"t. Oscar being "Ah"s-c"ah"r. The spelling would dissuade most from pronouncing Oscar as "Ah"s-Cur

@DylanDanger Good call, yeah: I can hear that accent in my head now, and I can see how they'd sound similar.

It's important that nobody in aviation uses the system of terms I came up with, Why Tsunami Phenoetic.

Additionally, "Fox" is the word used by NATO so signify the release of air-to-air-ammunition. While rather unlikely to occur in private aviation, it might yield a few question marks when used.

@Bobby I don't think you're helping your argument there as much as you think you are... the reason "Fox" is the word used there is precisely because that was the standard phonetic alphabet word for 'F' at the time the term was invented. I would assume they decided to use the phonetic alphabet for 'F' instead of saying "Fire" (which is what it stood for) to disambiguate "I'm firing an air-to-air weapon" from "My aircraft is on fire" in poor-quality radio communications.

The one syllable numbers pronounced as two syllable are 3, 4, 5, 9: thu-ree, fo-wer, fi-ev, and nin-er. The phone company, before dial phones were universal, used fiv-uh and ni-un.

@stretch ICAO nember are pronounced as Ze-ro, Wun, Too, Tree, Fow-er, Fife, Six, Sev-en, Ate, Nin-er, Ten

I find it quite odd that the phonetic alphabet is supposed to reduce confusion, and yet it purposely mispronounces certain words (e.g. "tree" instead of "three").

@pacoverflow in the case of three/tree the point of mispronouncing is to make the word sound more distinct over radio. "three" easily melts into "ee", tree maintains the sharp tr combo better. In aviation the word "tree" seldom used, for obvious reasons

I'd say this is very much "country-dependent". Maybe in UK-US, people say "FoxTrot" but in France, I think I never heard anything else than just "Fox"... (and everybody seems ok with that). But that was quite some time (years...) ago, any french pilot that could confirm?

@kebs the phonetic alphabet is an ICAO standard, there should be no variation in this among ICAO member states. Then agaim the french are famous for doing whatever ze 'ell zey please

Bad mics, speakers, ears, or radio connections can remove plosives, siblants, and basically all consonants, in which case the phonetic alphabet is heard as follows:
A-a, a-o, ar-ee, e-a, e-o, o-o, o, ou-e, i-i-u, u-i-e, i-o, i-a, aee, o-e-u, o-u, a-a, e-e, e-eeo, ie-a, a-o, U-ee-or, i-or, i-ee, e-ay, a-ee, u-u.
Vowels are the most likely part to be changed by accent, but the least likely part to be lost by the audio recording/transmission.
If you change "o-o" (foxtrot) to "o" (fox), there's nothing else there to distinguish it over a poor connection from "o" (golf).

Wow this explains how HoH hearing works, as well- why seemingly different words such as golf and fox can become indistinguishable. Vowels are king.

It's not acceptable. @Jpe61 explains very well about the phonetic alphabet's design, I will not cover that any more. There are other reasons that it's a bad practice:

If you shorten words or pronounce them incorrectly you introducing uncertainty, people hearing it may wonder whether the sender is shortening the words or if there is a problem with radio communication. They may waste time asking for clarification, or misinterpret your communication as a result
It makes people doubt your competence, and reduces others' confidence in you. If I hear someone making an effort with their calls there's a better chance they'll be equally conscientious in the rest of their piloting. Good phraseology can make the difference between getting an ATC clearance and them telling you to stay out of their zone

No, it is not acceptable. Even if all your radio transmissions were received with perfect audio fidelity and a minimum of RF interference, you still do not have a special right to give callsigns in a way that nobody else does. There are very good reasons why it is a uniform international standard that everybody, except you, has adhered to for decades, regardless of radio frequency & modulation type or aircraft class.
Sure, you can get away with it in some jurisdictions, & you say you hear others doing it, but you are advertising a level of sloppiness that many listening might perhaps apply to other aspects of your airmanship.
What prompted you to start abbreviating Foxtrot in the first place? Are you trying to sound casual, or sort of world-weary, ten-thousand-hours-nonchalant? Or did you hear others doing it & it sounded cool?
I've been flying since the 70s in a few different countries & using radios in a number of other industries & it's just never occurred to me to do this.

I heard it often at my home airport and remembered it as such, not really having to repeat it on most flights because I wasn't flying planes with an "F" on the tail. Not trying to make an exception for myself, quite the opposite.

A bit harsh @Mackk, the OP is asking to know what the right thing is, not to get justification for bad behavior.

Yeah, like what do you mean by "you can get away with it in some jurisdictions"?  Are there actual penalties?!

Is it a good thing,  no, but is it legal? Yes. There is no requirement for pilots to utilize standardized phraseology in the US. A controller may have to ask for a second read back if he/she doesn't understand your original one, which slows both the pilot and controller down. Controllers on the other hand do have to utilize standard phraseology if it is prescribed.


text:
Why would there be two RNAV waypoints co-located with a VOR?
Similar to this question but more so!
The Enroute H-5 chart shows two RNAV waypoints over Spinner VORTAC (SPI): SOHOW and USIRE. Neither waypoint appears on the Low IFR charts, nor on the VFR charts.

Enroute H-5 as of Feb. 25, 2021, via Skyvector

Searching the online Form 8260-2 database I see that Spinner is used for many different instrument procedures:

ROCKY SID off of Indianapolis
LAKES SID off of Kansas City
NATCA SID off of St Louis (an RNAV procedure!)
Several Victor airways and J-routes
Several instrument approaches to SPI and other nearby airports (including use as an IAF or feeder fix for RNAV approaches!)
MOTIF STAR into Chicago/Gary
Bradford STAR into Chicago
Braymer STAR into Kansas City
RIVRS STAR into St Louis
Vandalia STAR into St Louis

Some of these procedures only use Spinner to define a waypoint on the route, but in other procedures Spinner is actually flown over.
SOHOW does not have a Form 8260-2 listed in the database, but from a general search I notice it is used in:

LUCIT STAR into Gary

USIRE is used in:

KOLTS STAR into Indianapolis

Comparing the location of the three waypoints:

Waypoint
Latitude
Longitude

Spinner
39 50 23.04N
089 40 39.85W

SOHOW
39 50 25.37N
089 40 37.43W

USIRE
39 50 23.04N
089 40 39.06W

SOHOW is ≈300 feet northeast of Spinner
USIRE is ≈062 feet due east of Spinner
USIRE is ≈270 feet southwest of SOHOW

So the question is: Why do SOHOW and USIRE exist? And in particular why do they both exist? I can think of three reasons:

There is a rule saying that RNAV procedures must only use RNAV waypoints.
Counterargument: Spinner is a waypoint on the NATCA (RNAV) SID.
Spinner will be decommissioned soon, and the RNAV waypoints will be used instead. Neither waypoint is exactly on top of Spinner, but perhaps it doesn't need to be; the LBNON waypoint will replace Lebanon VOR/DME soon and those do not have the same exact coordinates.
Counterargument: Spinner is used in several VOR approaches and to define fixes on other instrument procedures; also I haven't heard of any plans to decommission it.
The specific configuration of the LUCIT and KOLTS arrival procedures necessitate the distinction between two points 270 feet away from each other (less than 0.05 nautical miles).
Counterargument: Really?


text:
VFR aircraft on a practice instrument approach—is the aircraft IFR? Do they receive separation?
A VFR aircraft contacts ATC and requests to execute a practice instrument approach (whether full-procedure or vectors-to-final). ATC issues the approach clearance, including the specific words "CLEARED [ry #] APPROACH."

Is the aircraft now operating under IFR?
What separation services does ATC provide?
What changes if ATC does not issue a "clearance" but instead an "approval?"

Answers for non-FAA jurisdictions also welcome. Related: Clearance limit of VFR practice IFR approach?, Can I make an ILS-Approach when I am VFR?, When are pilots required to have approval to conduct practice approaches?, Should I inform ATC about minor deviations to maintain VFR while on practice instrument approaches?

Also related: Can non-instrument rated pilots request practice approaches?, Is it necessary to switch from VFR to IFR when using ILS approach? [Ukraine]

Is the aircraft now IFR?
No.
The only way ATC can initiate an IFR clearance is by saying the words "CLEARED TO [location]" (7110.65 4–2–1b). The location they say is the clearance limit. If a pilot hears "CLEARED TO" they should know that they are receiving IFR separation (from both aircraft and terrain) and it is legal for them to enter clouds and other areas of reduced visibility.
ATC can also "clear" aircraft to execute an instrument approach procedure. This clearance does NOT change the flight rules status of the aircraft: IFR aircraft remain IFR and VFR aircraft remain VFR. If a VFR pilot receives an instrument approach clearance they still must comply with the applicable cloud clearance, ceiling, and visibility requirements which govern VFR flight. When initiating vectors to a practice instrument approach, ATC will specifically tell the pilot to "Maintain VFR" (7110.65 4–8–11a5), as a reminder that "even though ATC is providing IFR‐type instructions, the pilot is responsible for compliance with the applicable parts of the CFR governing VFR flight."
What separation services are provided?
Modified IFR separation.
When ATC "clears" a VFR aircraft for an instrument approach, they must provide modified IFR separation to the aircraft beginning "at the point where the approach clearance becomes effective" (4–8–11a2). "Modified IFR separation" means that the aircraft is provided any applicable separation as outlined in the rest of the .65: runway separation, lateral separation, non-radar separation, radar separation, visual separation, or vertical separation. The only exception is that, instead of requiring 1000' vertical separation between most other IFR aircraft, the minimum is 500'. (1000' is still required if either aircraft is a heavy or super.) For an IFR aircraft, an approach clearance automatically includes authorization to fly the published missed approach; this is not true for a VFR aircraft, which must receive prior authorization in order to be provided separation services when flying the missed (4–8–9).
What if ATC issues an "approval" only?
No separation services are provided.
If ATC does not "clear" a VFR aircraft for a practice approach, they must make this very explicit: "(Aircraft identification) MAINTAIN VFR, PRACTICE APPROACH APPROVED, NO SEPARATION SERVICES PROVIDED" (4–8–11a3). The aircraft may have been talking to ATC, may have been given vectors on to final approach course, and may even still be talking to ATC (if they are going in to a towered airport)—but no separation services are provided.
Why will ATC do one or the other?
Whether ATC issues a "clearance" or "approval" is prescribed the 7210.3 paragraphs 10–4–5 (for Terminal facilities) and 6–4–4 (for Enroute facilities). VFR pilots can expect to receive modified IFR separation when conducting practice approaches at the "primary airport" of an approach control, and at secondary airports "to the extent possible within existing resources." A Letter to Airmen should be published describing the type of services provided at a given airport, if any are.

My observation is that in regulatory airspace, they tell VFR practice approaches that they are "cleared" and when they're out of regulatory airspace, they say "practice approach approved".

@StephenS this will clarify: https://aviation.stackexchange.com/questions/23709/what-is-the-difference-between-regulatory-and-nonregulatory-airspace

@Ryan do you often do practice approaches to airports lying within MOAs? Did you perhaps mean "non-terminal-area airspace?" Remember that Class E/G are still "regulatory."

Yes, basically. However, G may not be considered regulatory (AIM 3-1-1).  The ones for which they say P.P.A. vs cleared are G at the surface vs E+ at the surface.  That's just my own observation.  I have no reference that it is guaranteed to work that way everywhere. No, I don't often do practice approaches into MOAs; I don't think I have ever.

@randomhead I regret that I have but one upvote to give. Thanks!

@StephenS Ah, but you can have two! Upvote question and answer.


text:
Existing EASA modules 1 to 13, missing module 3, EASA or UK CAA
I have all EASA modules for B2 aside from module 3. I live in the UK, and obtained these modules from 2017 to 2020 at a Part 145. Due to some circumstances, I didn't do the module 3 (electric fundamentals) until now.
I'm aware I need module 3 to become qualified. Do I do this as UK CAA exam? Also, does the gap mean I have to redo all other modules under UKCAA? Thanks.